Recently it has been discovered that arthropods mount effective inducible immune responses that resemble the innate immune responses of mammals. Thus, arthropod-borne bacteria must cope with the immune systems of both an invertebrate vector as well as a vertebrate host. The arthropod immune response is beginning to be characterized in model insects such as Drosophila. We plan to use this work as a model to characterize the immune response of medically important arthropods that transmit bacterial pathogens, namely fleas and ticks. Initial efforts will focus on: 1) purifying and characterizing antimicrobial peptides of fleas and ticks 2) determining their spectrum of activity, and 3) cloning and characterizing the genes that encode them. A practical outcome of this research could be the discovery of new clinically useful antimicrobials. The phagocytic hemocytes of fleas and ticks will also be characterized. Long-term goals are to determine the role arthropod immunity plays in vector competence. For example, after Borrelia spirochetes enter the tick?s gut in a blood meal, the spirochetes must penetrate the gut wall and migrate through the tick body cavity to the salivary glands in preparation for transmission to a new mammalian host. During this process the spirochetes would be expected to be exposed to the tick?s immune response, but how they avoid it is unknown. Once the normal antibacterial immune components of the tick are characterized, the mechanisms by which Borrelia spirochetes are able to overcome them can be examined. - Athropod immunity, Insect immunity, Innate immunity, Antimicrobial peptides, Hemocytes, Borrelia, Yersinia, Fleas, Ticks